This invention relates to the soldering of electrical components for surface mounting to printed circuit boards and more particularly to the processing of the components prior to such mounting.
At present there are two primary methods of joining electronic components to a printed circuit board. In one method, referred to as the leaded through-hole method, the components have wire leads protruding therefrom. The leads are placed through holes in the board and soldered to the circuit on the opposite side of the board.
The other more recent method involves mounting of the components directly onto the surface of the circuit board and is called the surface mount method. In this technique a solder paste is applied to the circuit board to form a desired pattern and the components are placed on the solder paste pattern and attachment of the components to the board is accomplished by a reflow process. The solder paste may be applied through a fine mesh screen or a thin metal stencil. The solder paste may also be dispensed through a small diameter tube in a desired pattern directly onto the circuit board.
In the surface mount approach solder pre-tin must be applied to untinned components before they are placed on a circuit board in order to fora a reliable solder connection at each point of contact between the component and the circuit board. In the case of leadless chip carriers (LCCs), pre-tinning of the component must be uniform across all of the attachment pads of the component in order to obtain uniform and controlled solder fillets at the board/lead interface. Numerous attempts have been made to pre-tin LCCs and obtain a uniform pre-tin. Robotic workcells designed for pre-tinning components have failed to produce consistent solder disposition with flat, four sided components such as LCCs.
Prior art FIG. 1 is a side view of a leadless chip carrier 10 with attachment pads 12 and pre-tin layers 14 and 14'. The LCC was pre-tinned by a standard wave soldering method. The relative direction of movement of the LCC is indicated by arrow A. In this method the carriers are placed in a pallet which moves from one end of a work cell to the other as the solder wave traverses the surface of the pads. Due to the unequal surface tension between the solder wave and the last set of attachment pads on the pallet, excess solder adheres to those pads.
As shown in FIG. 1, the pre-tin layer 14 on the pads 12 which are on the leading and midsection portions of the carrier (considered in the direction "B" of movement of the pallet) are even, but the pre-tin layer 14, on the pads 12 which are on the trailing portion of the carrier are uneven and have excess solder deposits. An operator then removes the excess solder by hand using a soldering iron. In many cases, too much solder is removed and the result is an LCC with too little solder on those pads.
It is therefore an object of the present invention to provide an apparatus which forms uniform pre-tin across all of the attachment pads of leadless chip carriers.
It is an additional object of the present invention to provide a method of pre-tinning leadless chip carriers which eliminates operator rework to remove excess solder following a pre-tin process.
It is a further object of the invention to provide a method of pre-tinning leadless chip carriers which is simple to perform and economical in cost.